1. Field of the Art
The present invention relates generally to a rotor assembly incorporated in a Roots pump, and more particularly to a technique for securely mounting a rotor made of light alloy on a support shaft made of steel.
2. Related Art Statement
A commonly known pump of a Roots type uses a plurality of rotor assemblies each of which includes a rotor, and a support shaft for supporting the rotor. The rotor and the shaft are fixed to each other such that the shaft is press-fitted in an axial bore formed concentrically through the rotor, while a lock pin is inserted through the rotor and the shaft in a direction intersecting the axis of the rotor assembly. Usually, the rotor is formed of a comparatively soft, light alloy material such as aluminum alloy for reduced inertia, while the support shaft is formed of a steel material for sufficient rigidity.
3. Problems Solved by the Invention
In a rotor assembly constructed as discussed above, the rotor and the support shaft have a relatively large difference in the thermal expansion coefficient. Accordingly, the rotor shrinks to a greater extent than the support shaft when the rotor assembly is cooled as in a thermal cycle shock test, wherein the rotor and the shaft are subject to a considerable change in temperature. As a result, the amount of interference between the inner surface of the rotor and the outer surface of the shaft is increased as compared with the nominal or predetermined suitable amount of interference given upon press fitting engagement of the shaft with the bore in the rotor. The increased amount of interference results in an increased stress (tensile stress) exerted to the rotor in its circumferential direction. The tensile stress may exceed the yield strength of the rotor material, causing plastic deformation of the rotor during cooling of the rotor assembly. Consequently, when the rotor assembly is subsequently exposed to a higher temperature, the amount of interference between the rotor and the shaft is reduced because of the plastic deformation, and the fastening force or surface pressure between the two members is accordingly reduced. This may permit a slight degree of relative rotational rattling movement between the rotor and the shaft in operation of the pump. While rotational and axial movements of the rotor relative to the shaft are inhibited by the lock pin, the pin holes in which the lock pin is inserted may be enlarged due to wear since the drive torque is transmitted to the rotor through the lock pin. Therefore, relative movements between the rotor and the shaft may take place if the press-fit force therebetween is reduced below the critical lower limit.
The above-indicted lock pin which bears the rotor drive torque is positioned at an axially middle portion of the rotor assembly, while the drive torque is imparted to the support shaft through a timing gear which is fixed to one of opposite ends of the shaft. Thus, there is a considerably distance between the timing gear and the lock pin, which may more or less cause a twisting of a portion of the shaft due to a torsional force applied to that portion. This is a cause for another inconvenience of the known Roots pump that a predetermined relative angular phase of the plurality of rotor assemblies may be lost during operation of the pump.
The inconveniences indicated above may give rise to an interference between the adjacent rotor assemblies, which leads to reduction of durability of the pump.